The invention relates to the field of permanent magnet structures, and more particularly to the fabrication of permanent magnet core elements using hot isostatic pressing techniques for densification of the magnetic core materials. The invention is particularly advantageous in the construction of radially oriented magnet elements for use in miniaturized traveling wave tubes (TWTs). The unique method and apparatus according to the invention provides for the complete and leakproof sealing of a permanent magnet core assembly in fabrication process, after the degassification step, but prior to the hot isostatic pressing procedure, without the application of heat to effect the closure, thus insuring the elimination of adverse effects that closure steps involving heating, hot welding, or the like could, and would, most likely produce.
The utilization of permanent magnet structures and devices to replace electromagnetic type yokes in electronic apparatus, cathode ray tubes for instance, has received significant acceptance in the electronics industry. Precision and miniaturization which is attainable with permanent magnet type structures and assemblies is, for the most part, not attainable with the use of electromagnetic structures. Permanent magnets made according to the present invention find particularly advantageous application where the focusing of an electron beam in a given apparatus is a critical factor. Such devices include traveling wave tubes and extended interaction amplifiers which, in turn, find application in microwave/millimeterwave communications, radar, and jamming apparatus for military and national security use.
It has been found through experience in this area that radially oriented cores are most beneficial and frequently essential to the miniaturized periodic permanent magnet stack assemblies used. It has been further found that a preferred means of fabricating these structures is by the hot isostatic pressing technique. This method has been found superior to alternative and somewhat traditional methods of densification of cores such as sintering and the like procedues, where the structure is vulnerable to fracture and sometimes even severe cracking, rendering the product-in-process completely useless. The reasons for this are within the knowledge of artisans practicing the technology; the basic causes being due to extremely high strains introduced upon cooling of the elements which have been heated for the densification sintering and the like steps. Where different materials are used, there is, of course, the further problem of anisotropy in the thermal expansion coefficients of several different materials employed in fabricating or assembling the apparatus. A major disadvantage and problem with hot isostatic pressing techniques, prior to the time of the present invention, has been the effective accomplishment of degassification, and then the effective sealing off of the container being used in the process, without damaging the magnets being made. Heretofore, whenever some sort of heat sealing has been used after the degassification step, the product has been put at risk because of the detrimental effects of such heat.
The technique of fabricating permanent magnet elements through the methods of hot isostatic pressing generally has been known in the art. Traditionally, the steps of permanent magnet fabrication involve the vacuum melting and chill casting of some alloy of magnetic material, followed by a comminution of grinding of the alloy until it is milled and or hydrided to the preselected partical size and morphology for the desired end product.
The particalized material is thereafter disposed in a die press wherein it is isostatically compressed to the required degree of densification.
Traditionally, after the desired degree of densification had been attained, further steps in the manufacturing process including hipping, followed by heat treatment, and/or sintering followed by heat treatment. The cores or individual magnet elements thus produced are then machined to the desired configurations and tested for integrity by means of X-ray diffraction or metallography beam microscopy.
The rings would then be assembled into the desired magnetic elements, impulse magnetized, and measured for magnetometry evaluation or achievement. Radial field evaluations and transverse field distribution measurements are then made, in the nature of quality control checks.
The problems encountered with the prior art technique of manufacturing permanent magnet structures in accordance with the foregoing description were that the end products are less than completely satisfactory in any instance where the heat applied to the material in process after its densification would tend to bring about cracking, spalling, or other forms of magnetically detrimental phenomena to the discs and core pieces which were to be the major elements of the finished magnet.